Low noise amplifier having improved linearity

ABSTRACT

Embodiments of the present general inventive concept include a low noise amplifier and method with an improved linearity while reducing a noise disadvantage (e.g., increase). One embodiment of a low noise amplifier can include a first transistor to receive an input signal at a control terminal thereof, a second transistor having a first terminal coupled to a second terminal of the first transistor, an envelope detector to output a control signal corresponding to a characteristic of the input signal and an envelope amplifier to amplify the control signal to be applied to a control terminal of the second transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 (a) from KoreanPatent Application No. 10-2006-0105463, filed on Oct. 30, 2006, in theKorean Intellectual Property Office, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to a low noise amplifier, andapparatuses and methods for the same.

2. Description of the Related Art

FIG. 1 is a diagram schematically illustrating a general transceivercircuit. As illustrated in FIG. 1, the transceiver circuit includes abase band processor 10, a DAC (digital-to-analog converter) 12, anup-conversion mixer 14, a power amplifier 16, a duplexer 18, an antenna20, a low noise amplifier 22, a down-conversion mixer 24, a variablegain amplifier 26, a filter 28, an ADC (analog-to-digital converter) 30and a local oscillator 32.

In FIG. 1, since the low noise amplifier 22 is a first amplifier in areceiving path, a noise figure or noise amount of the low noiseamplifier 22 is directly added to a noise figure or amount of an entirereceiving path. Therefore, among the amplifiers 22 and 26 in thereceiving path, the low noise amplifier 22 affects the noise figure ofthe entire receiving path the most. Therefore, the low noise amplifier22 in particular should be designed to have a small noise figure.

In the low noise amplifier 22, linearity is an important factor since asingle-tone desensitization and a cross modulation interference mayoccur because of a non-linearity. For instance, when an AMPS (AdvancedMobile Phone System) channel having a high signal level exists adjacentto a CDMA (Code Division Multiple Access) receiving channel that is adesired signal (e.g., spaced apart by 900 KHz), the AMPS channel acts asa single-tone interferer on the CDMA receiving channel. When thenon-linearity exists in the low noise amplifier 22, a sensitivity forthe CDMA receiving channel, which is the desired signal, is reduced by asignal of the AMPS channel (e.g., a gain is reduced). Such phenomenon isreferred to as the single-tone desensitization.

In addition, when a CDMA transmitting channel signal leaks to the lownoise amplifier 22 from the duplexer 18, the cross modulationinterference can occur between the CDMA transmitting channel signal andthe single-tone interferer. The cross modulation interference has abandwidth corresponding to a bandwidth of a CDMA transmitting channelcentered at the single-tone interferer. Therefore, the cross modulationinterference can generate an interference of the CDMA receiving channeladjacent to the single-tone interferer.

To suppress the single-tone desensitization and the cross modulationinterference (e.g., described above), the linearity of the low noiseamplifier 22 should be improved. One easy method to improve thelinearity of the amplifier is to increase a bias current. However, whenthe bias current is increased, a power consumption is increased, whichdecreases a lifespan of a battery of a mobile device. Therefore, amethod for reducing or minimizing the power consumption while improvingthe linearity is required.

An example of the method is disclosed by a paper “Vincent W Leung,Junxiong Deng, Prasad S. Gudem, and Lawrence E. Larson, Analysis ofEnvelope Signal Injection for Improvement of RF AmplifierIntermodulation Distortion, IEEE Journal of Solid-State Circuits, Vol.40, No. 9, September 2005, pp. 1888-1894” wherein the bias current isminimized when an input signal is small and the bias current isincreased when the input signal is large. An amplifier and an operationthereof disclosed in the paper are shown in FIGS. 2 a and 2 b,respectively.

As illustrated in to FIG. 2 a, the amplifier additionally includes aenvelope detector 32 compared to a conventional amplifier. The envelopedetector 32 detects a mean signal power and adjusts a bias current of atransistor 34 according to the detected mean signal power. When a powerof an input signal increases, the bias current of the transistor isincreased, whereby a load line is changed as shown in FIG. 2 b. As aresult, when the power of the input signal is small, an amplification iscarried out using a small bias current. When the power of the inputsignal is large, an amplification is carried out using a large biascurrent. Therefore, a mean power consumption is reduced.

However, while the amplifier of FIGS. 2 a-2 b has a high linearity and alow power consumption, the amplifier has various disadvantageous. Forexample, the noise figure of the amplifier is increased. Morespecifically, since the output of the envelope detector 32 is connectedto an input terminal of the transistor 34, a noise added by the envelopedetector 32 such as a thermal noise is amplified and outputted to affectthe noise figure of the amplifier. For at least such reasons, theamplifier is mainly used for a power amplifier such as power amplifier16 where the noise figure is relatively unimportant.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

An object of the application is to solve at least the above problemsand/or disadvantages or to provide at least the advantages describedherein in whole or in part.

Another object of the application to provide a low noise amplifierhaving a high linearity, or a low power consumption and/or a low noisefigure.

To achieve objects and/or utilities of embodiments of the application inwhole or in part, there is provided a low noise amplifier that caninclude a first transistor to receive an input signal at a controlterminal, a second transistor configured to couple a first terminal to asecond terminal of the first transistor and to provide an amplifieroutput signal, an envelope detector to output a signal representative ofthe input signal and an envelope amplifier to amplify the signal of theenvelope detector to apply to a control terminal of the secondtransistor.

To achieve objects and/or utilities of embodiments of the application inwhole or in part, there is provided an amplifier that can include acascode amplifier to receive an input signal, the cascode amplifierconfigured to include a common source amplifier and a common gateamplifier and an envelope detector to apply a representative signalcorresponding to a signal power of the input signal to a controlterminal of the common gate amplifier.

To achieve objects and/or utilities of embodiments of the application inwhole or in part, there is provided a receiving circuit that can includea low noise amplifier to amplify an input signal, a mixer todown-convert an output signal of the low noise amplifier, a variablegain amplifier to amplify an output signal of the mixer, a filter tofilter an output signal of the variable gain amplifier and an ADC toconvert an output signal of the filter to a digital signal, wherein thelow noise amplifier can include a cascode amplifier to receive an inputsignal, the cascode amplifier configured to include a common sourceamplifier and a common gate amplifier, an envelope detector to apply arepresentative signal corresponding to a signal power of the inputsignal to a control terminal of the common gate amplifier and anenvelope amplifier coupled between the envelope detector and the controlterminal of the common gate amplifier, the envelope amplifier to amplifythe representative signal to be applied to a control terminal of thecommon gate amplifier.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a diagram schematically illustrating a general transceivercircuit.

FIGS. 2 a-2 b are diagrams illustrating a related art amplifier and anoperation thereof.

FIG. 3 is a diagram illustrating an embodiment of amplifier inaccordance with the application.

DETAILED DESCRIPTION OF EMBODIMENTS

Exemplary embodiments according to the present general inventive conceptwill now be described in detail with reference to the accompanieddrawings. The interpretations of the terms and wordings used indescription and claims should not be limited to common or literalmeanings. Exemplary embodiments of the application are provided todescribe the general inventive concept more thoroughly for those skilledin the art.

FIG. 3 is a diagram illustrating an embodiment of an amplifier inaccordance with the application. As shown in FIG. 3, the amplifier caninclude a common source amplifier 42, a common gate amplifier 44, anenvelope detector 46 and a envelope amplifier 48. In addition, theamplifier may further include a first inductor 50 and a second inductor52.

The common source amplifier 42 and the common gate amplifier 44 arepreferably interconnected to form a cascode amplifier. An input signalcan be applied to a gate of the common source amplifier 42, and anoutput signal can be outputted from a drain of the common gate amplifier44.

The envelope detector 46 can measure a mean signal power of the inputsignal, and output a signal corresponding to the measured mean signalpower. The envelope detector 46 may simply be embodied by a diode 54 anda low pass filter 56. However, embodiments of the application are notintended to be limited by such an exemplary disclosure.

The envelope amplifier 48 can amplify and output the signal beingoutputted from the envelope detector 46. The term “envelope” of theenvelope amplifier 48 is used in a meaning that the signal outputtedfrom the envelope detector 46 is amplified, not to represent a specialmeaning. For example, a general amplifier or the like may be used as theenvelope amplifier 48. The envelope amplifier 48 may be an invertingamplifier, or a non-inverting amplifier. It is preferable that a gain ofthe envelope amplifier 48 is set such that a gain β of an envelope pathdescribed herein satisfies prescribed parameters. Thus, it is preferablethat a gain of the envelope amplifier 48 is set such that a gain β of aenvelope path satisfies equation 3, which is described herein. In orderto achieve this, the envelope amplifier 48 may be a variable gainamplifier.

The first inductor 50 can be coupled between a drain of the common gateamplifier 44 and a first power supply Vdd to provide an output impedancematching.

The second inductor 52 can be coupled between a source of the commonsource amplifier 42 and a second power supply Vss to provide a noisematching and/or an input impedance matching.

Operations of an amplifier according to the application will now bedescribed. As illustrated in FIG. 3, a large signal equation of thecommon source amplifier 42 may be expressed as equation 1.

$\begin{matrix}{I_{DS} = {\frac{1}{2}\mu_{n}C_{OX}\frac{W}{L}\left( {1 + {\lambda \; V_{DS}}} \right)\left( {V_{GS} - V_{TH}} \right)^{2}}} & \left\lbrack {{Equation}\mspace{20mu} 1} \right\rbrack\end{matrix}$

where I_(DS) is a drain current of the common source amplifier 42, μ_(n)is a mobility of an electron, C_(OX) is a capacitance of an oxide filmper unit area, W is a width, L is a length, λ is a channel lengthmodulation coefficient, V_(DS) is a voltage between a drain and thesource of the common source amplifier 42, V_(GS) is a voltage betweenthe gate and the source of the common source amplifier 42, and V_(TH) isa threshold voltage of the common source amplifier 42.

Assuming that an input small signal is v_(in), and an envelope signalβv² _(in) is applied to the drain of the common source amplifier 42 bythe envelope detector 46 and the common gate amplifier 44, a smallsignal drain current i_(DS) of the common source amplifier 42 may beexpressed as equation 2.

$\begin{matrix}\begin{matrix}{i_{DS} = {{k\left( {1 + {\lambda \; \beta \; v_{in}^{2}}} \right)}\left( {{g_{1}v_{in}} + {g_{2}v_{in}^{2}} + {g_{3}v_{in}^{3}} + \ldots} \right)}} \\{= {{{kg}_{1}v_{in}} + {{kg}_{2}v_{in}^{2}} + {\left( {{kg}_{3} + {k\; {\lambda\beta}\; g_{1}}} \right)v_{in}^{3}} + \ldots}}\end{matrix} & \left\lbrack {{Equation}\mspace{20mu} 2} \right\rbrack\end{matrix}$

where

${k = {\frac{1}{2}\mu_{n}C_{OX}\frac{W}{L}}},$

β is the gain of the envelope path (a path from an input of the envelopedetector 46 to a source of the common gate amplifier 44), and g₁, g₂ andg₃ are a first order coefficient, a second order coefficient and a thirdorder coefficient of a polynomial respectively, when a non-linearity ofan output to an input of the common source amplifier 42 is expressed asthe polynomial.

In order to improve the linearity of the amplifier in accordance withembodiments of the application, the third order coefficient is verysmall and preferably set to zero. Therefore, β may be expresses asequation 3.

$\begin{matrix}{\beta = {- \frac{g_{3}}{\lambda \; g_{1}}}} & \left\lbrack {{Equation}\mspace{20mu} 3} \right\rbrack\end{matrix}$

When β satisfies equation 3, the linearity of an amplifier in accordancewith the application is increased or optimally improved since a thirdterm is removed. When polarities of g₁ and g₃ are identical, theenvelope amplifier 48 should be the inverting amplifier since β is anegative number. When the polarities of g₁ and g₃ are different, theenvelope amplifier 48 should be the non-inverting amplifier since β is apositive number. For example, in case of an N channel MOSFET(Metal-Oxide Semiconductor Field Effect Transistor), the polarities ofg₁ and g₃ are identical in a moderate inversion region, and thepolarities of g₁ and g₃ are different in a strong inversion region.

In FIG. 3, the output signal may be outputted from the drain of thecommon gate amplifier 44. However, embodiments of the application arenot intended to be limited by such an exemplary disclosure. For example,the output signal may be outputted from the drain of the common sourceamplifier 42. It is understood by the skilled in the art that equations1 through 3 may be similarly applied.

In FIG. 3, while the N channel transistor (e.g., MOSFET) is used toembody the common source amplifier 42 and the common gate amplifier 44,embodiments of the application are not intended to be limited by such anexemplary disclosure. For example, the common source amplifier 42 andthe common gate amplifier 44 may be embodied using a p channeltransistor or the like.

As described, embodiments of an amplifier in accordance with the presentgeneral inventive concept have various advantages. For example,embodiments of an amplifier or methods thereof can concurrently satisfyhigh linearity and/or low power consumption since the third ordercoefficients may be set to zero or substantially zero without increasinga large amount of the drain current. For example, in a method where thebias current is increased to improve the linearity, a twice largercurrent should be provided in order to improve the linearity of 3 dB.However, since embodiments of an amplifier in accordance with theapplication can improve the linearity by setting the third ordercoefficient to zero instead of increasing the bias current, a largeamount of current is not required.

In addition, contrary to the related art amplifier, embodiments of anamplifier or methods thereof include advantages in that the noise figureis not sacrificed or increased. More specifically, the related artamplifier shown in FIG. 2 a is disadvantageous because the noise figureis greatly increased by the envelope detector since a noise generated inthe envelope detector is inputted to an amplifier by the transistor 34.However, in accordance with embodiments of an amplifier and methodsthereof, an output of an envelope detector is not inputted to the inputterminal of the common source amplifier 42. For example, an output of anenvelope detector can be used to control the common source amplifier 42(e.g., a drain voltage) so that the noise figure is not increased or notgreatly increased.

Moreover, embodiments of an amplifier in accordance with the applicationhave an advantage because such amplifiers may be used as a low noiseamplifier included in a receiving circuit.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with any embodiment, it is submitted that it is within thepurview of one skilled in the art to affect such feature, structure, orcharacteristic in connection with other ones of the embodiments.Furthermore, for ease of understanding, certain method procedures mayhave been delineated as separate procedures; however, these separatelydelineated procedures should not be construed as necessarily orderdependent in their performance. That is, some procedures may be able tobe performed in an alternative ordering, simultaneously, etc.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the spirit and scope of the principles ofthis disclosure. More particularly, various variations and modificationsare possible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

1. A low noise amplifier, comprising: a first transistor to receive aninput signal at a control terminal; a second transistor configured tocouple a first terminal to a second terminal of the first transistor andto provide an amplifier output signal; an envelope detector to output asignal representative of the input signal; and an envelope amplifier toamplify the signal of the envelope detector to apply to a controlterminal of the second transistor.
 2. The amplifier in accordance withclaim 1, wherein the envelope detector comprises: a diode to receive theinput signal; and a low pass filter to receive an output of the diode.3. The amplifier in accordance with claim 1, wherein a gain of anenvelope path β of the envelope amplifier is${\beta = {- \frac{g_{3}}{\lambda \; g_{1}}}},$ where λ is a channellength modulation coefficient, g₁ and g₃ are respectively a first ordercoefficient and a third order coefficient of a polynomial when anon-linearity of an output to an input of the first transistor isrepresented as the polynomial.
 4. The amplifier in accordance with claim1, wherein an output terminal of the low noise amplifier is configuredto be a second terminal of the second transistor or a second terminal ofthe first transistor.
 5. The amplifier in accordance with claim 1,comprising a first inductor coupled to a second terminal of the secondtransistor and a first power supply.
 6. The amplifier in accordance withclaim 5, comprising a second inductor coupled to a first terminal of thefirst transistor and a second power supply.
 7. The amplifier inaccordance with claim 1, wherein the control terminal, the firstterminal and the second terminal are respectively a gate, source anddrain electrode of the transistor, wherein the first and secondtransistors are MOSFET transistors, and wherein the signalrepresentative of the input signal corresponds to a mean power signal ofthe input signal.
 8. An amplifier comprising: a cascode amplifier toreceive an input signal, the cascode amplifier configured to include acommon source amplifier and a common gate amplifier; and an envelopedetector to apply a representative signal corresponding to a signalpower of the input signal to a control terminal of the common gateamplifier.
 9. The amplifier in accordance with claim 8, wherein theenvelope detector comprises: a diode to receive the input signal; and alow pass filter to receive an output of the diode.
 10. The amplifier inaccordance with claim 8, further comprising a envelope amplifier coupledbetween the envelope detector and the control terminal of the commongate amplifier, the envelope amplifier to amplify the representativesignal envelope to be applied to the control terminal of the common gateamplifier.
 11. The amplifier in accordance with claim 10, wherein a gainof the envelope amplifier is set such that a gain of an envelope path βsatisfies a equation ${\beta = {- \frac{g_{3}}{\lambda \; g_{1}}}},$where λ is a channel length modulation coefficient, g₁ and g₃ arerespectively a first order coefficient and a third order coefficient ofa polynomial when a non-linearity of an output to an input of the commonsource amplifier is expressed as the polynomial.
 12. The amplifier inaccordance with claim 11, wherein the common source amplifier comprisesa first transistor with a control terminal to receive the input signal,a second terminal coupled to the common gate amplifier and a firstterminal coupled to a first power supply through a first inductor, andwherein the common gate amplifier comprises a second transistor with afirst terminal coupled to a second terminal of the first transistor anda second terminal coupled through a second inductor to a second powersupply, wherein the output signal is output from the second terminal.13. A receiving circuit comprising: a low noise amplifier to amplify aninput signal; a mixer to down-convert an output signal of the low noiseamplifier; a variable gain amplifier to amplify an output signal of themixer; a filter to filter an output signal of the variable gainamplifier; and an ADC to convert an output signal of the filter to adigital signal, wherein the low noise amplifier comprises, a cascodeamplifier to receive an input signal, the cascode amplifier configuredto include a common source amplifier and a common gate amplifier, anenvelope detector to apply a representative signal corresponding to asignal power of the input signal to a control terminal of the commongate amplifier, and an envelope amplifier coupled between the envelopedetector and the control terminal of the common gate amplifier, theenvelope amplifier to amplify the representative signal to be applied toa control terminal of the common gate amplifier.
 14. The receivingcircuit in accordance with claim 13, wherein the envelope detectorcomprises: a diode to receive the input signal; and a low pass filter toreceive an output of the diode.
 15. The receiving circuit in accordancewith claim 13, wherein a gain of the envelope amplifier is set such thata gain of a envelope path β satisfies a equation${\beta = {- \frac{g_{3}}{\lambda \; g_{1}}}},$ where λ is a channellength modulation coefficient, g₁ and g₃ are respectively a first ordercoefficient and a third order coefficient of a polynomial when anon-linearity of an output to an input of the common source amplifier isexpressed as the polynomial.
 16. The receiving circuit in accordancewith claim 15, wherein the signal representative signal corresponds to amean power signal of the input signal.